From from the Indo-European root sed- (to sit) that is also the source of sit, chair, saddle, assess

From Latin assiduus, from assidere (to attend to, to sit down to), from ad- (toward) + sedere (to sit). Ultimately from the Indo-European root sed- (to sit) that is also the source of sit, chair, saddle, assess, sediment, soot, cathedral, and tetrahedron.

USAGE:

"The reason for his presence there [a Donald Duck statue in a temple garden] remains a mystery despite the author's most assiduous inquiries." Jeff Kingston; Chiang Mai: Thailand's beguiling Rose of the North; The Japan Times (Tokyo); Jun 28, 2009.